text:
On the LAANC map for sUAS operations, why does the permitted ceiling drop to zero southeast of KGFL?

Above is a screenshot1 from the LAANC map for obtaining authorization to operate sUAS (model airplanes, "drones", etc), centered on KGFL, which is Floyd Bennett airport near Glens Falls NY.  Why does the permitted ceiling drop to zero AGL southeast of KGFL?  Even though the permitted ceiling is as high as 400' AGL between this area and the airport?  This configuration is very unusual.

Above is a screenshot of the sectional chart for this area.  The airspace in question is E2 Class-E-to-surface airspace.
Would aircraft likely be operating under Instrument Flight Rules in IMC at very low altitudes (well below 400' AGL) in the airspace SE of KGFL?  Are there aspects of the instrument approaches into or out of KGFL that would justify banning all sUAS operation in the airspace 5 to 10 miles southeast of KGFL, even though there are many other airports with similar long arms of E2 Class-E-to-surface airspace where the permitted ceiling is all the way up at the maximum value of 400' AGL?
Are there any other similar cases in US airspace where as one travels in a straight line away from an airport, remaining within E2 Class-E-to-surface airspace which encompasses only a single airport with associated instrument approaches, the permitted ceiling indicated by the LAANC system increases, and then decreases again?
It's probably safe to assume that the zero-ceiling designation is not to protect the small airport "Argyle" (1C3).

After all, in the above zoomed-out screenshot of the sectional chart from this area, we can count 33 other airports, none of which are protected by LAANC grid squares.
The following screenshots of the airspace around KSHR shows the FAA's more typical treatment of E2 Class-E-to-surface airspace: the areas distant from the airport have a 400-foot ceiling on the LAANC grid.

Here's a screenshot of the sectional chart showing KSHR:

Below are screenshots of the sectional chart and LAANC grid for another airport -- KTHV -- that has three outlying airports within the E2 Class-E-to-surface airspace.  Note that the permitted ceiling is 400' AGL in the vicinity of these outlying airports.  Reinforcing the point that the zero-ceiling grid squares southeast of KGFL are unlikely to be designated for the purpose of the protecting the airspace of the small airport "Argyle" (1C3).

So what is the reason for the unusually large area of zero-permitted ceiling on the LAANC grid SE of Glens Falls airport?
Footnotes:

All LAANC screenshots taken 12-23-20, and still were current as of 2-3-21.

I’m voting to close this question because it is about Drones (belongs on Drones.SE) and not about aviation per se.

I’ve noticed that the LAANC ceiling around airports does generally encompass the area for the standard traffic pattern. Could this be the key to why it is set to 0 on the Southeast Class E extension for KGFL? Like you said, it could be  protecting the smaller airport.

@quietflyer - Yes... this is off-topic on Av.SE. Per the Help Center guidance. So it might be migrated, or simply cross-posted, or just closed here without ever showing up there. Any of those are fine. But there is no aspect of this question that brings it within the realm of this exchange.

This question looks perfectly fine to me.  It would also be on topic on Drones.SE.  Either one would be fine I think.  I look forward to reading the answers.

There is nothing authoritative in that Meta q/a. The Help Center is authoritative, and it's pretty clear. Aircraft, aircraft pilots, aircraft design, etc. Drones have their site, which is where this belongs, and the overlap isn't that much.  Certainly not this.

This question should be re-opened. Here's why -- https://aviation.meta.stackexchange.com/q/4155/34686 .Consider voting to re-open.

Reopen because... you posted your viewpoint on Meta??? That's not how Meta & reopening closed questions works!

@RalphJ and nobody posted a good reason why not. sure, not enough time has passed, but posting on meta is exactly the way it can work.

@Federico for future reference, how much time should ideally pass?  (We've probably been through this once before but I guess I forgot.)

Btw, just in case anyone cares, I was literally within minutes of hitting "send" on the major edit w/ the screenshots etc when the question got closed; the edit was not primarily motivated by a desire to change the question to get it reopened, just to improve it generally.

@RalpJ -- see this meta answer, and the associated comment by ymb1  -- https://aviation.meta.stackexchange.com/a/4059/34686

This seems like a general aviation question to me - it is unlikely FAA made these changes in consultation with drone enthusiasts, so it is likely to be for reasons of general aviation.

@quietflyer usually we said to wait for the meta question to have been up for at least a week

@Federico -- I'm not quite sure I follow-- did you mean to say, wait till the non-meta question has been closed for at least a week, to post the meta question?

no, you wait for the meta question to have been opened and seen for at least a week before saying that there is any kind of consensus on meta that implies anything for the non-meta question

But I should feel free to post my own answer to the meta question right away, correct?  I didn't mean to say anything about a consensus on the meta question.  The comment here was just to direct people to the meta question including my "pro" argument, not to imply that a consensus had already been reached.  Maybe it was unclear.

Gotta say, I think the community consensus on this question is becoming abundantly clear... the lack of up-votes, reopen votes, and answers is exactly what one might expect for a question about a type of chart that aircraft pilots don't use and don't have reason to care, or even know, about.

@RalphJ -- Still pretty early in the process. Meta question hasn't even been up for 12 hours yet. See Federico's last comment above.  As of this moment, there have only been 22 views of the meta question ( https://aviation.meta.stackexchange.com/questions/4155/should-the-question-on-the-laanc-map-for-suas-operations-why-does-the-permitte ).  ASE is a tiny community, and things take time to happen.

@RalphJ  -- PS , sUAS pilots are aircraft pilots.  That's what the A in sUAS means, after all.  So says the FAA.

It turns out that the E2 Class-E-to-surface airspace around KGFL has been recently expanded, and the long "arm" projecting to the southwest is a new addition.  KGFL is listed in the current (as of February 2021) version of the FAA's "Airspace Designations and Reporting Points" document, FAA Joint Order 7400.11E.  The airspace change was published in the Federal Register on July 31, 2020, and became effective November 5, 2020.  The old airspace description may be seen on page E-30 of the July 21, 2020 edition of the Airspace Designations and Reporting Points document, J.O. 7400.11E.
One might wonder whether, after the airspace change, the newly-added LAANC grid squares were assigned a zero ceiling to accommodate some unusual feature of the new instrument approaches and departures in the vicinity, while by some oversight no adjustment was made to adjust the ceilings of the previously existing grid squares to accommodate the new approaches and departures.
Close scrutiny of the approach plates suggests that this is unlikely.  Granted, there is some modest amount of relief in the terrain southeast of KGFL. The ridge east of the town of "North Argyle", about 9.6 miles from the airport, tops out at just over 1000' MSL-- let's call it 1100' MSL to be conservative. The airport KGFL is at 328' MSL, so by our conservative estimate the top of the ridge is 772' higher than the airport. This works out to a slope of 66 to 1, or 0.87 degrees above horizontal. Add 400' to the top of the ridge to represent the maximum AGL altitude that the FAA typically grants for sUAS operations, and we have a 43.4 to 1 slope to the airport, or 1.32 degrees above horizontal.  It is difficult to see how permitting sUAS flight beneath this less-than-2-degree-slope would endanger aircraft on the instrument approaches into KGFL. For example, the RNAV (GPS) RWY 30 approach appears to show a Final Approach Fix ERUPE at 2300' MSL just 5 nautical miles from the airport. The Intermediate Fix CATIR is at 4200' MSL, 12 nautical miles from the airport.
Further scrutiny has revealed that the zero ceiling assigned to the new LAANC grid squares southeast of KGFL perfectly fits a pattern, that appears to have little relationship to safety considerations.
The pattern is as follows:  for most or all expansions or other modifications of existing controlled airspace effective 9/10/20 or later, any new squares that have to be added to the LAANC grid have been assigned a ceiling of 0 AGL.
As a result, whenever a significant new "arm" or projection is added to an existing area of controlled airspace, then we can get a pattern like we see at KGFL, where the portion of the new projection that lies outside of any previously existing LAANC grid squares is assigned a ceiling of zero, while the (older) grid squares closer to the airport retain a higher ceiling.
On the other hand, when an existing area of controlled airspace is expanded by a small amount, then we can have a pattern where just a few new LAANC grid squares, seemingly randomly scattered around the periphery of the old airspace, need to be created to accommodate the expanded airspace.  Very often the new airspace just barely impinges into most of these new grid squares.  All of these new grid squares are assigned a ceiling of zero.  This can happen even when the airspace is not actually expanded at all, but rather is simply moved a few hundred feet due to a slight change in the official location of the airport.  It's pretty clear that no safety consideration could possibly be involved in the zero-ceiling assignment in the latter case.
Why this pattern has recently arisen, remains a mystery.  At present it would be hard to say whether it is due to a decision taken at a high level of the FAA, or due to the choices of some particular data entry technician, or due to exigencies of life in the time of Covid-19.  Is there an intention to revisit these cases and correct the arbitrarily low ceiling assignments?  Is there an intention to continue the practice indefinitely?  The FAA sUAS Help Desk has been contacted, and further developments may ensue.
Here are additional specific examples of the pattern described above:1

KDRO -- Durango-La Plata County Airport -- Durango CO -- E2 Class-E-to-surface
Old airspace description page E-72 in FAA Joint Order 7400.11A,new airspace description on page E-68 in Joint Order 7400.11E.  Effective 09/10/20, the official airport location was moved about 130' to the NW, but more significantly, a new "arm" was added to the Class E airspace, projecting 2 miles to the NE, 2 miles in width.  As a result of this new "arm",  7 new squares were added to the LAANC grid, of all which assigned a ceiling of zero.   Note the presence of grid squares with a 400' ceiling between the new zero-ceiling grid squares and the airport.

KDLN -- Dillon Airport -- Dillon MT -- E2 Class-E-to-surface -- second attachment
Old airspace description page E-77 in 7400.11A, new airspace description page E-74 in 7400.11E.  Effective 9/10/20, the Class E circle was decreased in radius by 0.9 miles, but a new "arm" was also added, 4.8 miles in width and projecting .7 miles beyond the original radius.   As a result of this new "arm", 8 new squares were added to the LAANC grid, all of which were assigned a ceiling of zero.  Again, note the presence of grid squares with a 400' ceiling between the new zero-ceiling grid squares and the airport.

KCXO -- Conroe-North Houston Regional Airport -- Houston, TX -- Class D --third attachment
Old airspace description page D-125 in 7400.11A, new airspace description page D-125 in 7400.11E.  Effective 9/10/20, the official location of the airport was shifted about 350' to the NNW, and the radius of the Class D circle was increased by .7 miles (about 3700').  As a result of these changes, 27 new  squares were added to the LAANC grid, all of which were assigned a ceiling of zero.  Again, note the presence of grid squares with a 400' ceiling between the new zero-ceiling grid squares and the airport.

KFWS -- Fort Worth Spinks Airport -- Fort Worth, Texas -- Class D --fourth attachment
Old airspace description page D-129 in 7400.11A, new airspace description page D-125 in 7400.11E.  Effective 9/10/20, the official location of the airport was shifted about 85 feet to the west.  As a result of these changes, a single new square was added to the LAANC grid, barely penetrated by the new location of the controlled airspace, with an assigned ceiling of "0" AGL.  Again, note the presence of grid squares with a 400' ceiling between the new zero-ceiling grid square and the airport.

KALN --  St Louis Regional Airport  --Alton/St Louis, IL -- Class D
Old airspace description is on page D-31 in 7400.11E, new airspace description is on this page of the Federal Register.
page D-125 in 7400.11E.  Effective 11/05/20, the Class D airspace was expanded by .2 miles (about 1056 feet), and the center of the circle was shifted about 218' to the southwest due to the change in the official location of the airport.  As a result, there are now 6 anomalous zero-ceiling LAANC grid squares running from the south edge of the airspace clockwise around to the northwest edge of the airspace.  Note the presence of grid squares with a 400' ceiling between the new zero-ceiling grid squares and the airport.

PABE --  Bethel Airport-- Bethel, AK-- Class D
Old airspace description is on page D-3 in 7400.11E, new airspace description is on this page of the Federal Register.
page D-125 in 7400.11E.  Effective 12/31/20, the Class D circle was shifted about 430' to the southeast due to the change in the official location of the airport.  As a result, there are now 3 anomalous zero-ceiling LAANC grid squares at the southeast edge of the airspace.  Note the presence of grid squares with a 400' ceiling between the new zero-ceiling grid squares and the airport.

PASI2 -- Sitka Rocky Gutierrez Airport -- Sitka, AK -- E2 Class-E-to-surface airspace
Old airspace description is on page E-9 in 7400.11E, new airspace description is on this page of the Federal Register.  Effective 11/05/20, the Class-E-to-surface circle was shifted about 150' to the southeast due to the change in the official location of the airport.  As a result, there is now a single anomalous zero-ceiling LAANC grid squares at the ESE edge of the airspace.  Note the presence of grid squares with a 400' ceiling between the new zero-ceiling grid squares and the airport.
Footnotes:

All LAANC screenshots taken on 2-1-21

As of 2-1-21 this link to an on-line portrayal of the aviation sectional chart covering PASI still showed the airspace as it was configured before a recent change that came into effect 11-5-20.  It appears that the 11/5/20 edition of the Juneau sectional chart has not yet actually been made available on-line (see non-working links for Juneau sectional on the FAA's "VFR Raster Charts" page.)  The new configuration of the airspace at PASI, which involves a (nearly circular) E2 Class-E-to-surface area and a single, very large, rectangular E4 Class-E-to-surface extension, may be seen by selecting the appropriate layers on the LAANC map display after entering "PASI" into the search window.  The airspace configuration at PASI therefore is now similar to the airspace configuration at KACV and KTLV, as described in the related ASE question What is the primary reason that the non-towered airports KACV and KTVL have E4 extensions to E2 airspace? .

Hopefully the FAA answer sheds more light on this!

@UnrecognizedFallingObject -- the particular case of KGFL has now been fixed-- LAANC now shows a 400' ceiling for the grid squares in the arm of controlled airspace projecting off to the southeast.  The other cases haven't been changed yet.  At some point I'll update the answer.


text:
In a full-rudder sideslip in a Schweizer 2-22 or 2-33 glider, is the descent rate higher at 50 mph airspeed than at 60 mph airspeed?
Is it really true that in a full-rudder sideslip in a Schweizer 2-22 or 2-33 glider, the descent rate is higher at 50 mph airspeed than at 60 mph airspeed?
Likewise, is it really true that in a full-rudder sideslip in a Schweizer 2-22 or 2-33 glider, the no-wind glide path is steeper at 50 mph airspeed than at 60 mph airspeed?
I've heard several instructors in my glider club assert these things.  Some allege that at higher airspeeds the fin overpowers the rudder and reduces the obtainable slip angle (as measured by the yaw string).
I've logged plenty of time in these types of gliders but never have attempted to explore this issue experimentally.  But my intuition is that the slip angle (as measured at the yaw string) is not significantly larger at a lower airspeed than a higher airspeed, and that a given slip angle (as measured at the yaw string) produces much more drag (and also much more sideforce, requiring a larger bank angle to compensate) at a higher airspeed than at a lower airspeed.  Therefore I would expect the descent rate to be higher at a higher airspeed, and I would also expect the glide path to be steeper at a higher airspeed.
The minimum sink rate airspeed and the best L/D airspeed in these gliders are both in the neighborhood of 40 mph.
Bear in mind that the no-wind glide path is determined by the ratio of Lift to Drag, which is very nearly equal to the ratio of Weight to Drag.  So the question about the glide path is simply a question about which configuration creates more Drag.  Also, it is clear that if the glide path in the full-rudder sideslip is in fact steeper at 60 mph than at 50 mph, then the sink rate cannot be higher at 50 mph than at 60 mph.
Since the purpose of the full-rudder slip is to steepen the glide path and get the glider on the ground closer to the beginning of the runway, the glide angle is actually the parameter of greatest interest, but for whatever reason, in discussions between instructors and students I've heard the assertion phrased in terms of sink rate just as often as in terms of the actual glide path or glide angle.

Full disclosure: I have exactly 3 hours logged in a glider and have never specifically flown the 2-22 or 2-33.
That said, I do have some experience and understanding of slips, and the principles should be the same.To answer your question, I will draw from quotes taken from this article on Boldmethod.com.

In a forward slip, the amount of slip, and therefore the sink rate, is determined by the bank angle. The steeper the bank is, the steeper the descent.

However, there will be a limit to how much bank angle you can introduce before you can no longer hold a constant ground track.

In light airplanes, the steepness of forward slips and sideslips is limited by the amount of available rudder, or the rudder-limit. You may reach a point where full rudder is required to maintain heading.

If you are already at this point, with full rudder deflection, maximum allowable bank, and a constant ground track, there’s only one variable left - Pitch. Pitching down increases speed and increases descent rate.

As you lower the nose, airspeed and descent rate will both increase. The additional airspeed will give your rudder more effectiveness, allowing you to increase the slip. The opposite is true of raising the nose.

This means that you are absolutely correct. Properly configured, a higher airspeed should give you a higher descent rate.

Fin and rudder are one unit, creating an airfoil.  Any aerodynamic effect of this unit, on the center of gravity, will be greater at higher airspeed.  Since min sink rate is around 40 mph, lift/drag, there for glide angle should be much steeper at 60 mph.  Wing AOA will be less at 60 mph, but total drag greater.
The Schweitzer 2-22 design features a large bulbous nose (to aerodynamicly counter balance the rear fuselage) making the rudder more effective.  Some gliders feature forward swept wings for the same reason.
The shape of the nose may create a higher anti-rudder torque at 60 mph.  Fuselage "lift/thrust" has been cited as a contributory factor in spins as well.  As this nose is yawed to the side, airflow over its rounded surfaces may be enough to start working against the rudder at higher airspeeds.
The other "anti rudder" effect may come from the high wing design, where the fuse may shield some of the downwind wing, creating more torque on the upwind wing at higher speeds.
A third possibility is at 60 mph the AOA of the wing may be "blanking" the rudder, or more downwash may be striking it at 50 mph.
But actual flight testing may resolve this issue.  Are spoilers deployed?
It may turn out 50 mph is better because of lower airspeed after slip is removed, prior to landing.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
Why does a steady-state non-zero body yaw and/ or roll rate induce side force?
An answer on ASE states

The presence of steady-state non-zero body yaw and roll rate induces side force ($C_{y_p}$ and $C_{y_r}$).

What is a non-mathematical explanation of why this is so?
Is it correct to say that the side force components described above would exist even if the body were somehow following the same trajectory, and experiencing the same yaw and roll rotation rates, in a vacuum?  I.e., that they do not directly stem from aerodynamic effects?
Also, do the sideforce components created by a non-zero body yaw and/ or roll rate depend upon at what physical location on the aircraft we are measuring the sideforce, i.e. upon exactly where the slip-skid ball (inclinometer) is located?  If so, might there be a location where the measured sideforce resulting from the yaw and/or roll rate is zero even though the aircraft is yawing and/or rolling?

Let's say we have a thin, straight and rigid stick traveling in a 2D circular path. We will attach our reference of frame to the CG of the stick, aligned with its natural forward. If it has a forward speed of $u$ and the circular path has a radius of curvature of $R$, then it has a yaw rate, $\dot{\psi}=u/R$, all of which are expressed in the said reference frame.

At locations forward or aft of the CG, different parts of the stick will experience different lateral speeds due to the yaw rate, when measured using our reference frame: $u(x) = \dot{\psi}x$. The intuition is that the stick is spinning, so top and bottom will experience the largest speed while the centre will have none.
If you had attached a fin near the of the stick, then it will experience a side airspeed, effectively giving it an aerodynamic incidence angle. As a result, there will be aerodynamic forces and moments, simply from the motion of rotation.
From an analysis perspective, we usually don't re-calculate the effects on every surface due to local speed changes. Instead, the stability derivatives are measured in the body axis (well, usually in the stability axis but easily transformed to the body axis). So we can easily lump the sum of the aerodynamic effects of the individual portions into a single derivative with respect to the rate of rotation about that axis.

The original answer quoted in my question also referenced yaw damping, which would seem to me to related to the aerodynamic torque associated with the aerodynamic sideforce generated by specific parts of the aircraft, like the fin, having a sideways velocity with respect to the direction of the flight path.  But I'm still not seeing how a steady (as opposed to increasing or decreasing) yaw rate automatically generates a sideforce.  I'll make an edit to the question that may possibly help express the source of my confustion a bit better.

@quietflyer Of course there would be no aerodynamic forces/moments in a vacuum.

I just didn't understand whether the components were an inherent aspect of the rotation rates and trajectory, or something specifically generated by aerodynamic consequences following from the rotation rates/ trajectory.  And still don't understand which is the case, I guess.

@quietflyer It's all aerodynamic due to change in local airflow from the rotation. A constant rate of rotation, as mentioned, will induce different velocities at different parts of the body, relative to the air.

@Noah Car is different in that the slip angle is determined by the relative cornering of the front and rear wheels. Depending on the CG and arrangement, the total vehicle slip can be adverse or proverse.

@Noah You mean to their respective local radius of curvature? Yes, but your vane will have to be infinitesimal, otherwise it will experience local airflow that's different from the point of rotation you're trying to measure.

@Noah This is going way beyond the comment section. This should be posted as a new question, maybe on physics SE. The article is basically agreeing with this answer.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Why would the propeller of a lightsport plane or ultralight w/ Rotax engine be connected to the engine through a clutch, and not turn at low rpm?
Why would the propeller of an ultralight or lightsport plane be connected to the engine through a clutch, and not turn at low rpm?
I've often witnessed the engine start of a Bailey-Moyes Dragonfly (link to flight manual, links to cool you-tube videos but not showing engine start #1, #2) and the prop did not turn until the motor rpm was increased substantially above idle.  It was clear that some sort of automatic clutch was not engaging until the engine rpm was increased above idle.
It would seem that any sort of clutch that would allow the engine to run without the prop turning would add un-necessary complexity.  So why was this feature incorporated into the drive train of some versions of this aircraft?
I'm rather sure I was told the answer to this once, but I've forgotten.
I know that the aircraft in question was powered with a Rotax engine, but several different Rotax engines have been used on this type of aircraft, and at present I'm not able to identify the specific one I witnessed where the prop was not turning at idle.  It seems likely that many other aircraft powered by the same engine would use the same gearbox with the same automatic clutch feature.

This question could be improved by 1) identifying one or more of the specific engines exhibiting this feature, and 2) adding a link to a video showing engine start-up where it can be clearly seen that the prop is not turning until engine rpm is increased. Such videos must be widely available on the net.

Centrifugal clutches are mainly used to provide a compliant connection between the engine and prop to absorb power pulse peaks, as well as allow the engine to be automatically disconnected from the drive system at idle.
The big bugaboo of reduction drives over the years has always been torsional resonance resulting from the flywheel - the prop - being driven through a springy drive path by strong pulsating power impulses.  At RPMs where the pulse/feedback frequency matches the natural frequency of the drive system/prop, you get massive resonant torsional vibration, which at minimum is unpleasant and at maximum can trash the gear train.
Over years various attempts have been made to tame this phenomenon in a practical way.  It's not actually a good idea to use a centrifugal clutch in a normal aircraft.  If the engine quits, the prop can freewheel and this turns it into a very powerful airbrake. It's also a reliability issue, being a friction device in a critical power path.
That said, a coupling that has been used with some success, and even certified at one time, was the Flexidyne Dry Fluid Coupling, which works like a centrifugal clutch but uses steel shot encasing a wavy plate in the housing to act a bit like a fluid torque converter.  The FAA certified Molt Aerocar used a Flexidyne to connect the Lycoming to the prop down a long drive shaft (which suffers from the same kind of torsional resonance problems).  Problem is, Flexidynes get really hot, and you still have the windmilling problem (Molt Taylor used the Flexidyne in several of his homebuilt concepts but he never really addressed the freewheeling prop issue) and a propeller brake might be in order if you want to be able to glide (or a feathering or folding prop).
You see centrifugal clutches used in paramotors, mostly because it allows the motor to be running at idle without the prop turning, a handy feature for a paramotor.  I wouldn't be surprised if the Dragonfly's engine is actually using a Flexidyne instead of a friction clutch.
The most common compliant coupling between motor and prop however, is rubber.  Most commonly a rubber drive belt system, used everywhere on ultralights and some automotive aircraft conversions.  Or as successfully used by Rotax, a rubber coupling in the gearbox itself. Rotax, after a lot of effort, was able to create a rubber drive isolator that would be effective in a given engine's operating range.  This would have taken a massive engineering effort over the years to perfect.

In addition to xxavier's answer, a clutch makes it easier to maually start the engine (without the prop load) and easier to idle (for the same reason).  There is also a safety benefit as well:
If you have ever seen a video of someone desperately hanging on to a groundlooping Piper Cub that was hand started with the prop, at too high a throttle setting, you'll know what I mean.  A simple centrifugal clutch, that does not engage the propeller until the engine rpm is raised significantly, is a good idea.  These are common on pull-cord started chainsaws and go-carts.
Having a second person assisting you with the Piper Cub is a good idea too.

If I remember correctly, the 4-stroke Rotax engines include a sliding clutch to protect the crankshaft from shocks in case the prop accidentally contacts the ground.

It would also be for torsional resonance issues as well.


text:
What is the purpose of downthrust?
Many light general aviation aircraft-- and perhaps many other aircraft as well-- are designed with some of amount of downthrust in the engine mounting, relative to the centerline of the fuselage.  Presumably the wing incidence angle is chosen to streamline the fuselage in some particular flight regime.  So why is any downthrust designed into the engine mount?  Exactly what is the designer trying to optimize, by including downthrust in the design?  In what undesirable way would the aircraft's flight characteristics be different if the downthrust were not there?
For example, is the intention that adding power (without re-trimming the elevator) should not make the aircraft climb?  Or that adding power should not cause a change in trim airspeed?  Or that adding power should not cause a change in trim angle-of-attack?  Would there be a "wrong-way" change in trim airspeed when power was added, if the downthrust were not present?  If so, why?
Note that one reason for some reduction in trim airspeed when power is added to climb but angle-of-attack is held constant, is given in this related answer.  (See the tables-- and note that a similar reduction in trim airspeed would also occur when power was reduced to descend.)  The present question anticipates that this effect may be minor enough not to be the main reason for including downthrust in the design of any particular airplane.
This question is aimed primarily at aircraft with nose-mounted propellers and conventional tailed configuration, so please be sure to address this configuration, but feel free to discuss other configurations as well.

This is a real question not a "trap" question; I have some idea of what the answer might be but wouldn't be able to offer a definitive answer w/o some further research!

I think you'll find that if you look at the thrust axis in flight at normal cruise AOA, as opposed to the thrust axis relative to the longitudinal axis of the plane itself, you'll find that it's more or less horizontal in level cruisiing flight.  If wing incidence is zero, and the engine is canted down 3 deg, and the wing operates at 3 deg AOA in cruise flight, there's your horizontal thrust axis with no significant UP or DOWN component to it.

@JohnK -- thanks for the note, but would the wing incidence really typically be zero?  This does not seem optimized for streamlining the fuselage at cruise speed.  Why would the wing incidence not be set to streamline the fuselage at cruise speed? So I understand your contention is that fuselage is typically not level at cruise and that's why the downthrust is there, to make the thrust line parallel to the flight path at cruise?

@JohnK-- my belief is that at least in the case of high-wing single-engine airplanes, the downthrust is there to counteract a tendency for the wing's downwash to strike the tail and pitch the nose up to a higher angle-of-attack, especially at higher power settings, since the downwash is accentuated by the propwash.  But I'd like to hear it from someone who has better knowledge of this either from actually working in a/c design, or from more extensive research than I've done at present.

With regards to John's comment, we should remember the other player applying torque to the Center of Gravity, the horizontal stabilizer/ elevator/ stabilator/ trim tab.  Ideally both the thrust line and the horizontal stabilizer will be 0 degrees to the horizon with the wing at optimal cruise AOA.
Under power, relaxing static stability with a little competing downthrust will lessen the need for excessive trimming any time thrust is changed, and also leaves static stability full on when the plane powers down to land.
This is a very safe and widely used application of downthrust for GA aircraft.  Because the cosine of the small downthrust angle is very small (cos 3 degrees = 0.9986), effect on the horizontal thrust component is minimal.

Most light aircraft sit slightly nose-up in flight. This simplifies the design parameters for the wing mounting, as the wing needs to angle slightly up. The designer seeks to maximise thrust by angling the engine down so it points straight forward.
On a high-wing type, angling the engine down also adjust the thrust line so it passes closer to the centre of pressure. This reduces trim changes with engine power.
However on a low-wing type this requires the engine to be angled upwards. But angling it up too much moves the thrust line lower still, so that in an engine-out situation the nose will drop to help maintain airspeed and avoid a stall. The angled thrust also introduces a vertical component, i.e. lift. This is at the expense of forward thrust. For a small angle, the loss of forward thrust and airspeed is insignificant, and the extra lift actually offloads the wing a little bit, allowing a reduction in drag so that the plane may actually fly a fraction faster. But for a significant angle the thrust loss increases more than the wing drag decreases and the plane slows down worse than before. So the angle should not be overdone.

It seems like you are suggesting that the typical situation involves upthrust, not downthrust.  Is this really true?

@quietflyer I take your point. I'll add something to my answer.

@Guy Inchbald "for a significant angle the thrust loss increases more than the wing drag decreases.  It gets good when the numbers are run.  How about a calculation for our generic GA 10/1 L/D ratio, 5 degrees up thrust? It would be sin (5) for vertical up thrust, cos (5) -1 for horizontal thrust loss.  But sin (5) wing deflection drag spoils it unless we keep lift vertical with leading edge slats.  +1 for some numbers.  (Slats may chip in even more lift from increased camber)!


text:
Were planned release dates skipped in 2020 for the VFR sectional charts for Juneau Alaska and Dutch Harbor Alaska?
Were planned release dates skipped in 2020 for the VFR sectional charts for Juneau Alaska and Dutch Harbor Alaska?
On February 3 2021, the FAA's "Raster Charts" webpage listed a March 26 2020 date for the latest edition of the Juneau sectional, and a February 27 2019 date for the latest edition of the Dutch Harbor sectional.  These dates seem unusually old.  (And interestingly, at that time, the links to those editions didn't even work.)
There was a rather significant airspace change at PASI at Sitka Alaska (which is located on the Juneau chart), effective November 5 2020, which appears to coincide with what should have been a planned release date for this chart.  The E2 Class-E-to-surface airspace at Sitka was changed from an "L" shape with arms to the northwest and southwest, to a nearly circular shape with a very large rectangular E4 Class-E-to-surface "extension" projecting toward the northwest.  As of February 3 2021, the current VFR sectional products available online still showed the old airspace configuration, while the new airspace configuration could be seen by going to the  LAANC map and typing "PASI" in the search window and enabling the E2 and E4 layers.  (Images of each attached below-- screenshots taken February 3, 2021.)
It seems odd that such a major change in airspace configuration would not have been covered by new edition of the VFR sectional chart.  Was a release of the Juneau chart planned for November 5, 2020, and then skipped for some reason?
Bonus: if so, for what reason?
The rather low "edition numbers" shown on the current edition of the FAA's "Raster Charts" webpage for many of the Alaska charts does suggest that they generally been released only about half as frequently as most other charts.  So maybe these long intervals have been planned, or at least did not come as a surprise.
Basically, in a nutshell, this question can be re-stated as "Did the latest editions of the Juneau and Dutch Harbor VFR sectional charts prior to the 2-25-2021 update show an 'expiration date' that was earlier than 2-25-21?  And if so, why was the previous planned update skipped?"
A note in closing -- The FAA's "Aeronautical Chart Bulletins" webpage is currently a good resource for seeing a list of significant changes that have occurred since the last publication of any given VFR aeronautical chart.  Also, as of February 3 2021, this webpage indicated that the Dutch Harbor chart was actually last updated on Feb 27 2020 rather than February 27 2019, so the 2-27-2019 "latest update" date on shown on the "Raster Charts" webpage on 2-3-21 may have been a typo.  That doesn't change the basic thrust of this question-- was a planned update in 2020 missed?

Re "Basically in a nutshell this question can be re-stated as "As of 2-3-21, do the most recent paper editions of the Juneau and Dutch Harbor VFR sectional charts show an 'expiration date' that is earlier than today's date? And if so, why were the planned release dates skipped?""-- this would be referencing the last edition before the 2-25-21 update.

Re "The current edition of the FAA's "Raster Charts" webpage lists a March 26 2020 edition for the Juneau sectional and a February 27 2019 edition for the Dutch Harbor sectional. "-- this statement was accurate when this question was posted, Feb 3 21.  It no longer is.

The FAA has just switched to a 56-day release cycle (double AIRAC cycle) for all VFR sectional charts. In the process of aligning the release cycle, they may have skipped an update. However, the chart of Dutch Harbor was on a cycle of one year before, and would have been due on 25th of February anyway. Juneau was also on a one-year schedule and would have been due on the 25th of March. It is very unlikely that a release would have been skipped.
The story behind the switch to the 56-day release schedule can be found in this FAA briefing. See Milestone #2 below, which may be the reason for skipping a release.

Background/Discussion: The lifecycles of FAA-produced Visual Flight
Rules (VFR) Charts vary from 168 days to two years and at one time
various charts did not update for years longer (Helicopter and Grand
Canyon for example).
Extended and unsynchronized lifecycles create an
undue burden on the NAS and on chart users. The NOTAM System must
carry many facility NOTAMs until consistently charted. As a result,
chart users are burdened with numerous NOTAMS and difficulty arises in
identifying pertinent NOTAMs and correctly applying them. Numerous
changes are alerted in Chart Supplement Chart Bulletins that many
users either do not know exist and/or do not find readily accessible.
Unsynchronized chart dates lead to inconsistent data capture on
overlapping areas, adjacent charts and other chart products.
Producing
56-day VFR Charts will provide significant relief to a number of these
issues. The NAS picture will be consistent with that reflected on
Enroute, Terminal and Supplemental products. NOTAMs will be
significantly reduced as charts will capture changes with every 56-
day AIRAC cycle. Chart Supplement Chart Bulletins will no longer be
necessary.
Planned Actions: Update the VFR Sectional, Terminal Area,
Flyway Planning and Helicopter Route Chart series on the 56-day AIRAC
cycle. Implement this effort across a series of milestones:
Milestone
#1 will synchronize the cutoff for VFR Charts with other aeronautical products. (June 18, 2020)
Milestone #2 will eliminate 28-Day AIRAC
date VFR Charts to align with airspace amendments and will stage
charts for 56-day AIRAC date production. (September 10, 2020)
Milestone #3 will realize full implementation of the 56-day update of
VFR Charts and elimination of supporting Chart Supplement Chart
Bulletins. (February 25, 2021)

And more history can be found here: VFR Chart Print Schedule Realignment and Synchronization


text:
In some "turn coordinators", does the gyro spin opposite to the direction it spins in a "turn rate indicator"?
In some "turn coordinators", does the gyro spin opposite to the direction it spins in a "turn rate indicator"?
A "turn coordinator" has a symbolic airplane on the display face that tilts (banks) to indicate the direction of yaw or roll.  The gimbal axis of the gyro is tilted up about 30 degrees to allow sensing of roll as well as yaw.  A "turn rate indicator" uses a needle on the display face to indicate the direction of yaw.  The gimbal axis of the gyro is parallel to the aircraft's longitudinal axis rather than tilted upwards, and only yaw is sensed.
In both instruments, the spin axis of the gyro wheel is oriented laterally, i.e. parallel to the aircraft's wingspan, when the aircraft is not moving.  We know that in all "turn rate indicators", and in at least some "turn coordinators", the gyro wheel spins clockwise as viewed from the right, so that the top of the gyro wheel spins away from the pilot.
In some "turn coordinators", does the gyro wheel spin in the reverse direction-- i.e. counterclockwise as viewed from the right, so that the top of the gyro wheel spins toward the pilot?
What is the advantage of having the gyro in a "turn coordinator" spin counterclockwise as viewed from the right, if any are in fact configured this way?
Or is it just an aviation "myth" that in some turn coordinators, the top of the gyro spins toward the pilot rather than away from the pilot?
The ideal answer would include video evidence or a manufacturer's diagram, etc.
This question is only about instruments using actual physical rotating gyros.

The gyro is angled at 45 degrees so that it's sensitive to both yaw and bank.Pure yawing motions have a limited effect, and pure banking motions have a limited effect, with the strongest effect from when the two movements are combined as in an actual turn.To help, the gyro has a hydraulic damper, like those tiny dampers for turntable tone arms, to help filter out small yaw and roll movements.The damper on mine crapped out once and the thing bounced around wildly, unusable.They are electric, running off fixed frequency ac created by a teeny tiny static inverter in the unit. Dunno the direction.

@JohnK do you happen to know if the needle-style turn rate indicator has the same kind of damper, or if it doesn't need it, why not?  Maybe grounds for another ASE question...

I had a turn and bank apart once and there was no damping device in it. On the DVs, I think some of the members on here think of questions posted as a method to post a prepared response by the poster as underhanded somehow, when it's actually fine to do.

@JohnK thanks for the info; would be interesting to learn someday why it is not needed

@quietflyer Unless this post made up said myth (which doesn't seem to be the case) I don't see why this is a poor question.

@Chris -- look through my (admittedly overly long) answer to see the links to three sources (including 1 ASE answer) repeating the wrong-way spin idea re turn coordinators.  Look for the stand-alone sentence "Here are three such sources:" about 2/3 of the way down through the answer.

Neither instrument works properly unless it turns clockwise as viewed from the right.
If a turn coordinator is made to rotate counterclockwise as viewed from the right, the gyro will rotate counterclockwise as viewed by the pilot when the aircraft is yawed or rolled to the left. If it is spinning the other way, it will rotate clockwise. Since the symbolic airplane could be made to rotate with or against the gyro, it doesn't really matter for yawing or rolling motions which way the gyroscope rotates.
The response to pitch is different- if the gyroscope rotates counterclockwise as viewed from the right then pitching up during a turn will cause the indicated turn rate will decrease. This is a big problem! In a 30° banked level turn, the yaw rate (that is, intrinsic rotation about the vertical axis) of is only 87% of the turn rate (that is, the change in heading). The rest is pitching up.
When the turn coordinator is calibrated to show a proper standard rate turn in a 30° bank, the pitching response makes up for the other 13%. If it rotated the other way, it would instead show 13% less and indicate a turn rate 26% too low!

@quietflyer For sure. I also discuss the pitching response of a turn indicator here. I appreciate your question- it made me think through the topics and I feel I understand the instruments a lot better now.

@quietflyer No, a CCW gyro causes the pitch to indicate in the wrong direction. Sure you could still calibrate it for one specific bank angle, but it would read grossly wrong at any other bank angle. Fundamentally a level turn always pitch up with the yaw so you need "more pitch" to indicate as "more turn" not "less turn" and you only get that with a CW gyro.

@quietflyer Also, the percentages I refer to are just referring to the motion. The yaw rate in a bank angle of $\theta$ is equal to the turn rate times $\cos\theta$. So you set the stiffness of the spring so that its pitch response is enough to make up for that $1-\cos\theta$ deficit in the yaw response. This only works exactly at one specific bank angle but with a CW gyro it's pretty close for a wide range of angles since the cosine function doesn't change very fast at small angles.

In short, in a 30 degree banked level turn your yaw rate is 87% of the turn rate ($\cos30^\circ$) and your pitch rate is 50% of your turn rate ($\sin30^\circ$). If you want to calibrate your instrument for a 30 degree bank then you make the stiffness of the spring whatever it needs to be so that in a standard turn in the neighborhood of 30 degrees is about enough to make up for the change in yaw rate. E.g. in a 29 degree bank you'll have more yaw and less pitch. If you calibrated the spring properly the loss in pitch response will be about equal to the gain in yaw response so it will be right.

In a standard rate turn the torque (and thus the deflection of the instrument) due to pitching and yawing motions combined is proportional to $\cos(\theta-\phi)$, where $\theta$ is the bank angle and $\phi$ is the angle the gyroscope makes with the vertical axis of the airplane in a standard turn. By setting $\phi$ to be a specific angle you make the turn coordinator most accurate in the vicinity of that angle, since the cosine function varies slowly around 0.

The same instrument with CCW spin has torque proportional to $\cos(\theta+\phi)$. There is no way you can calibrate this to have a similar "plateau" like $\cos(\theta-\phi)$ has when $\theta\approx\phi$.

@JuanJimenez -- care to comment on this answer, considering your opposing answer https://aviation.stackexchange.com/a/56126/34686 ?

(Some responses to above answer / comments now added in chat https://chat.stackexchange.com/rooms/150416/discussion-between-quiet-flyer-and-chris )

I'm going to go ahead and delete most of my comments above to help keep mods happy, see the chat instead--

On all turn rate indicators-- the instrument with the needle indicator and no tilt to the gyro's gimbal axis-- the gyro spins clockwise as seen from the right, with the top of the gyro spinning away from the pilot.  On several diagrams and videos of turn coordinators-- the instrument with the moving symbolic airplane on the face of the instrument and the tilted axis for the gyro gimble-- the gyro wheel is clearly set up to spin in the same direction.  At present the author of this answer has been able to find no diagrams or videos of a turn coordinator whose gyro wheel spins in the opposite direction.
In this image from Wikipedia, the gyro on a turn coordinator is illustrated as spinning clockwise as seen from the right, just like the gyro on the turn rate indicator in the diagram.
In this AOPA article, the gyro on a turn coordinator is illustrated as spinning clockwise as seen from the right.  The reverse gearing is illustrated that makes the symbolic airplane tilt to the right when the gyro spin axis tilts to the left, exactly as we see in the video below.  If the gyro were spinning the other way, the gyro's spin axis would tilt to the right when the aircraft yawed or rolled to the right, and no reverse gearing would be needed to make the symbolic airplane tilt to the right.
In this video of a Cessna Turn Coordinator, when electrical power is applied at 1:50, we can clearly see the gyro wheel start to spin clockwise as viewed from the right, with the top of the wheel moving away from the pilot.  When the instrument is yawed to the left at 3:33, we can see the spin axis of the gyro wheel tilt (bank) to the right-- clearly the symbolic airplane on the front of the instrument is being driven by reverse gearing, so it tilts (banks) the opposite direction as the spin axis of the gyro wheel.  This is all exactly as is the case in a Turn Rate Indicator, except for the fact that the gimble axis of the Turn Coordinator's gyro is tilted so that it can sense roll as well as yaw.
Here's another similar video "How a Turn Coordinator Works-- Inner Workings".  When the instrument is subjected to a left yaw rotation, the spin axis of the gyro wheel tilts (banks) to the right, and the symbolic airplane on the front of the instrument, driven through reverse gearing, tilts (banks) to the left.  We can't clearly see the gyro wheel's direction of spin in this video, but if it were spinning the opposite direction, the spin axis of the gyro wheel would tilt (bank) the opposite direction in response to yaw or roll
Here's another video showing more of the same.  The relevant portion start at 7:11.  The clockwise direction of rotation of the gyro wheel as viewed from the right is clearly illustrated, as is the direction that the spin axis of the gyro tilts (banks) in response to yaw or roll, and the way that the symbolic airplane on the front of the aircraft is geared to tilt (bank) in the opposite direction.
Here's a video of a Turn Rate Indicator.  More of the same, except that the gimbal axis of the gyro wheel is purely longitudinal rather than tilted upwards, and the instrument senses only yaw, not roll.   Again, we can't clearly see the gyro wheel's direction of spin in this video, but if it must be clockwise as viewed from the right-- if the gyro wheel were spinning the opposite direction, the spin axis of the gyro wheel would tilt (bank) the opposite direction in response to yaw, and the needle would need to be driven by direct gearing rather than by reverse gearing.
Note with direction of gyro rotation shown in these videos-- where when the spin axis of the gyro wheel tilts (banks) in once direction, the indicator on the face of the instrument tilts (banks) in the opposite direction-- in a normal coordinated turn, as the aircraft banks, the gyro's spin axis will tilt relative to the instrument housing and the airplane in the direction that keeps the gyro wheel's spin axis somewhat close to horizontal relative to the earth's surface.  If the gyro wheel were spinning the opposite direction and the gyro's spin axis were tilting (banking) the opposite direction in response to yaw or roll, the gyro wheel's spin axis would be tilting away from horizontal relative to the earth's surface.
Note that the direction of rotation of the gyro wheel that we see in the above videos and illustrations will cause the instrument to over-indicate the turn rate whenever the aircraft is under an unusually high G-load in relation to the bank angle, i.e. whenever the flight path is curving upward.  This is true for two different reasons.  (This is about to get complicated-- the reader is encouraged to make some sketches, or better yet, to make a simple model of a gyro wheel such as a pencil skewered through a round disk that he or she can hold in different attitudes in space.)  One, whenever the gyro's spin axis is banked off level relative to the instrument housing and the airplane, any pitch rotation of the aircraft is transformed by the gyroscopic precession effect into a tendency for the bank angle of the gyro's spin axis relative to the instrument housing and the airplane to increase.  (This may be more clear if we imagine the gyro's spin axis to be banked almost vertically, relative to the instrument housing and the airplane, which would never actually happen in reality).  This effect is also present in a turn rate indicator, where the gyro's gimbal axis is parallel to the aircraft's longitudinal axis.  And two, since the turn coordinator gyro's gimbal axis is not truly aligned with the aircraft's longitudinal axis, but rather is mounted with the forward end raised and the aft end lowered to permit sensing of roll rotation as well as yaw rotation, whenever the gyro's spin axis is banked away from level relative to the instrument housing and the airplane, the lowered end of the gyro's spin axis is moved forward, and the raised end of the gyro's spin axis is moved aft.  This re-orientation of the gyro's spin axis causes any nose-up pitch rotation of the aircraft to be transformed by the gyroscopic precession effect into a tendency for the gyro's spin axis to yaw even further away from the aircraft's lateral axis and closer to the aircraft's longitudinal axis.  (This may be more clear if we imagine that the  gyro's spin axis to be almost parallel to the aircraft's longitudinal axis, which would never actually happen in reality.)  Since the gyro's gimbal axis is mounted with the forward end raised and the aft end lowered, this yaw rotation of the gyro's spin axis must also cause the bank angle of the gyro's spin axis relative to the instrument housing and the airplane to increase.  This will cause the turn rate indication on the face of the instrument to increase.  This second effect is only present in a turn coordinator, and not in a turn rate indicator.
Yet one often comes across sources that say that the gyro wheel in a turn coordinator spins counterclockwise as viewed from the right-- so that the top of the gyro wheel is spinning toward the pilot.  The same sources say that in contrast to a turn rate indicator, a turn coordinator tends to under-indicate when the G-loading is excessively high in relation to the bank angle.  A common theme of these sources is that this characteristic makes it difficult to use the turn coordinator to recover from unusual attitudes.
Here are three such sources:
A post on an airline pilot's discussion forum (#13)
Another post on the same discussion forum (#2)
An ASE answer to a related question

Or is it just an aviation "myth" that on some turn coordinators, the
top of the gyro spins toward the pilot rather than away from the
pilot?

It seems unlikely.  The idea seems very widespread.  Certainly an under-indication of the turn rate when the G-loading is excessively high in relation to the bank angle is indeed what we would expect if the gyro wheel in a turn coordinator were spinning counterclockwise as viewed from the right-- so that the top of the gyro wheel were spinning toward the pilot.
Here's why:
One, whenever the gyro's spin axis is banked off level relative to the instrument housing and the airplane, any pitch rotation of the aircraft is transformed by the gyroscopic precession effect into a tendency for the bank angle of the gyro's spin axis relative to the instrument housing and the airplane to decrease.  (Again, this may be more clear if we imagine the gyro's spin axis to be banked almost vertically, relative to the instrument housing and the airplane, which would never actually happen in reality).  And two, since the gyro's gimbal axis is not truly aligned with the aircraft's longitudinal axis, but rather is mounted with the forward end raised and the aft end lowered (to permit sensing of roll rotation as well as yaw rotation), whenever the gyro spin axis is banked away from level relative to the instrument housing and the airplane, the lowered end of the gyro spin axis is moved forward, and the raised end of the gyro spin axis is moved aft.  This re-orientation of the gyro's spin axis causes any nose-up pitch rotation to be transformed by the gyroscopic precession effect into a tendency for the gyro's spin axis to yaw closer to the aircraft's lateral axis and further away from the aircraft's longitudinal axis.  (This may be more clear if we imagine the gyro's spin axis to initially be almost parallel to the aircraft's longitudinal axis, which would never actually happen in reality.)  Since the gyro's gimbal axis is mounted with the forward end raised and the aft end lowered, this yaw rotation of the gyro's spin axis must also cause the bank angle of the gyro's spin axis relative to the instrument housing and the airplane to decrease.  This will cause the turn rate indication on the face of the instrument to decrease.  Again, this second effect is only present in a turn coordinator, and not in a turn rate indicator.
As we've already noted, if the gyro wheel rotates in this direction-- counter-clockwise as seen from the right, so that the top of the gyro wheel spins toward the pilot-- in a normal coordinated turn, as the aircraft banks, the gyro's spin axis will tilt relative to the instrument housing and the airplane in the direction that brings the gyro's spin axis further away from horizontal relative to the earth's surface, not closer to horizontal relative to the earth's surface.   This would appear to make the calibration of the instrument at any given turn rate more dependent on the bank angle, which means that the calibration of the instrument at any given turn rate will be more dependent on True airspeed.  The reason for this is explored in more detail in this related answer.

What is the advantage of having the gyro in a "turn coordinator" spin counterclockwise as viewed from the right, if any are in fact configured this way?

Lacking a decisive answer to this question, here are a few thoughts.  We've already noted that when the gyro wheel rotates clockwise as viewed from the right, with the top of the gyro wheel spinning away from the pilot, two different effects combine to make the instrument over-indicate the turn or roll rate whenever the G-load is excessive for the bank angle.  Perhaps in some designs, this might tend to "peg" at the indicator at full deflection too easily.  Perhaps rotating the gyro the other direction helps to calm things down.  Perhaps superior damping technologies have made it no longer necessary to use this strategy.
Links (also cited in body of answer)--
Wikipedia illustration of Turn and Slip Indicator and Turn Coordinator
"How it works: Turn Coordinator".  Jill W. Tallman.  Flight Training (AOPA).  January 1, 2019.
Video links (also cited in body of answer)--
How Gyroscopic Turn & Slip Indicator Works (You Tube video) -- in the cutaway, note how far the gyro's spin axis tilts to the side (rolls) in response to yaw rotation-- the tilt at full deflection appears to be about 40 degrees.  We can see that the tilt is in the direction that would bring the gyro's spin axis closer to horizontal with respect to the earth during a normal coordinated turn.  By the fact that the gyro's spin axis rolls to the left when the instrument is yawed to the right, we can see that the gyro must be spinning clockwise as viewed from the right-- i.e. the top of the gyro is moving away from the pilot.
"How a Turn Coordinator Works-- Inner Workings"  -- Again we can see that gyro's spin axis rolls in response to yaw, and the roll is in the direction that would bring the gyro's spin axis closer to horizontal with respect to the earth during a normal coordinated turn.  Again, by the fact that the gyro's spin axis rolls to the left when the instrument is yawed to the right, we can see that in this particular turn rate coordinator, the gyro must be spinning clockwise as viewed from the right--i.e. the top of the gyro is moving away from the pilot.
Cessna Turn Coordinator (You Tube video) -- when electrical power is applied at 1:50, we can clearly see the gyro wheel start to spin clockwise as viewed from the right, with the top of the wheel moving away from the pilot.
"Gyroscopic instruments" (You Tube video) -- this animation is very well done.  The relevant portion starts at 7:11.  The clockwise direction of rotation of the gyro wheel as viewed from the right is clearly illustrated, as is the direction that the spin axis of the gyro tilts (banks) in response to yaw or roll, and the way that the symbolic airplane on the front of the aircraft is geared to tilt (bank) in the opposite direction.
Related ASE links--
During "partial panel" flying, in what ways is a turn rate indicator more useful than a turn coordinator?
Are turn rate indicators calibrated to a certain speed? If so, what speed?

"By the fact that the gyro's spin axis rolls to the left when the instrument is yawed to the right, we can see that the gyro must be spinning clockwise as viewed from the right-- i.e. the top of the gyro is moving toward the pilot." If it's rotating clockwise as viewed from the right, wouldn't the top of the gyro by moving away from the pilot?

@Chris -- re your comment -- yes, indeed.  Fixed.  And that's what I meant to write.  Haven't found any videos documenting a "wrong-way" spin (CCW as viewed from right, top of gyro moving toward pilot.)

Content added in chat here https://chat.stackexchange.com/rooms/150416/discussion-between-quiet-flyer-and-chris seems relevant. In short, very stiff centering springs would tend to transform the instrument into something close to a pure yaw rate indicator (at least, in case of turn coord, whenever roll rate is zero), regardless of direction of spin of the gyro wheel, which might be sufficient to explain the observed/claimed tendency of some tc's to under-read under heavy G-loading. Doesn't settle the fundamental question being asked as to did any tc's spin CCW as viewed from the right.


text:
Is there a standard term for "3D groundspeed"?
Does the term "groundspeed" always refer only to the magnitude of the horizontal component of an aircraft's velocity vector with respect to the earth, or does it sometimes include the contribution from the vertical component of the aircraft's motion?
If the latter, what are some examples (from outside sources) of the terminology being used this way?
If the former, is there a standard term for the magnitude of the aircraft's full three-dimensional velocity vector with respect to the earth, including the contribution from the vertical component of the aircraft's
motion?

Based on my interpretation of this question:  https://aviation.stackexchange.com/questions/83163/what-kind-of-airspeed-does-a-3d-gps-measure I think this is just a duplicate, with slightly different wording.  I would guess that the OP agrees, but will let him/her speak for themselves...

Nah, I won't raise any objection, have more interesting battles to pick, feel free to vote to close/ reopen however you feel so moved...

Ditto, I will leave it for others to decide...  ;)  But I do like Ralph's answer.

The former is correct; an F-16 climing straight up has 0 groundspeed.
"Inertial velocity" would accurately describe the 3D motion you're describing.

As suggested in a comment by ASE user "nanoman", the most appropriate term appears to be "inertial speed".


text:
At what angle-of-attack (sideslip angle) would a symmetrical vertical fin stop "helping" a deflected rudder produce sideforce?
For rudder deflection angles of 5, 10, 20, 30, and 45 degrees, at what (negative) angle-of-attack does a fin-rudder combination (including dorsal fin if present) create exactly the same amount of sideforce as would be the case if the fin somehow vanished, leaving the rudder to act as an all-moving vertical tail, with no change in its geometrical orientation relative to the aircraft?
For the purpose of this question, define angle-of-attack in relation to the chord line of the fin alone, not including the deflected rudder. If we ignore things like spiral slipstream, deflection of airflow around cabin, etc, it would be the same as the sideslip angle.
Assume a symmetrical airfoil on the vertical fin.
Assume the direction of sideslip (if sideslip angle is non-zero) is such that the airflow is striking the side of the fin that is opposite the direction of rudder deflection-- i.e. the rudder deflection is in the pro-slip direction.  The deflected rudder is promoting, not opposing, the sideslip.
Implicit in this question is the idea that at an angle-of-attack of zero relative to the fin, the fin-rudder combination creates slightly more sideforce than the deflected rudder would by itself, for any given rudder deflection angle.  I.e. at zero degrees angle-of-attack, and presumably at other very small angles-of-attack, the fin is actually helping the rudder to create sideforce.  This question is about the point where that stops being true.  However you may answer "zero degrees" if you feel that is the correct answer.  If you feel that the fin is actually reducing the net sideforce produced by a deflected rudder even at zero degrees angle-of-attack relative to the fin, then please make a note of that in your answer; in this case the question would have no correct answer.
To allow for a definitive answer, I'll specify to use the fin (including dorsal fin) and rudder from a current -production Cessna 172 S, but feel to post an answer addressing the fin and rudder from any other aircraft.
Approximations are fine-- obviously an exact answer would involve a complicated computer modelliing project, or actual wind tunnel experiments.  My hope is that existing information relating to the lift curves of symmetrical airfoils with deployed flaps may be adapted to give an approximation of an answer to this question.
Here is the basic purpose of this question: in general, during a strong sideslip (such as an intentional cross-controlled sideslip for a crosswind landing), the fixed vertical fin is creating a yaw torque that is opposing the yaw torque from the deflected rudder, and the sideslip angle would increase if the fixed fin were somehow to suddenly vanish.  However it seems likely that at very small sideslip angles, this is no longer true.  At a sideslip angle of zero-- which would still be compatible with a deflected rudder if some other yaw torque were acting on the aircraft-- it seems likely that the fin-rudder combination forms a better airfoil-- generating more sideways lift-- than the deflected rudder would by itself if the fin were absent.  This question is asking at what slip angle this no longer is true, for a given rudder deflection.  In other words, for a given rudder deflection, at what slip angle is the fixed fin neither adding to, nor subtracting from, the aerodynamic sideforce created by the deflected rudder.
Related questions and answers:
How is a sideslip maintained (aerodynamically)?
How is a sideslip maintained (aerodynamically)?
Note that the present question is different than the related question At what angle-of-attack (sideslip angle) would a symmetric vertical fin plus a deflected rudder have a lift coefficient of exactly zero? .  The related question asks about the conditions where the fin-rudder combination creates no lift at all.  Obviously in that case, if the rudder is deflected, the fin is producing a sideforce that acts against the sideforce form the deflected rudder.   The present question acts about the conditions where the sideforce created by the fin-rudder combination is exactly the same as the sideforce that would be created by the deflected rudder alone if the fin were to vanish.

What do you mean "help"? They are one single entity. A flap can't exist without a wing.

@tuskiomi - not a duplicate-- a deflected rudder with no fin attached would not have a lift coefficient of zero ( unless sideslip angle is same as rudder deflection angle).  A fin-rudder combination would be expected to have a lift coefficient of zero at a much smaller sideslip angle than the rudder deflection angle.  At higher slip angles the fin ends up "fighting" the rudder.

I still think that you can combine the two into one question

@Jimmy -- sure it could, and likewise a rudder can exist without a fin - see for example ruddercraft.com

@tuskiomi - yes they could be combined, but they were created to replace a deleted question where I was told in essence  "too many questions here".

@quietflyer Then it's just an all moving tail...in which case inc = sideslip - sidewash + deflection

I'm curious, what motivates the question? I'm not sure I understand the purpose behind the question here.

A diagram would help very much. As the question is, I do not understand what the crux is.

It might be some kind of "control reversal" effect with ailerons on a stalled wing, and larger flaps creating a pitching moment opposite of predicted effect (Cessna 172 pitches UP) when flaps deployed (due to break down of top rear lift on wing).  But this may not apply to empennage, as the tail assembly still will "lift", even though it's center of "lift" may move slightly with full rudder deflection.  But it does raise some interesting questions regarding the control reversal phenomena.  (which flaps may produce prior to "stall").  Since we were discussing "crow" on smaller models.

Downwash would also affect empennage, of course.

The question has interesting applications, as the concept can also be applied to horizontal stabilizer/elevator design.
First take a simple flat plate V stab and rudder, and line them up in the old wind tunnel.  At 0 angle of attack there is no side force, the assembly weather vanes straight into the wind.
Now deflect the rudder to the left.  A side force is created, pushing the tail assembly to the right, until the forces left and right rebalance.  Note it is the V stabs "job" to prevent excessive slip, analogous to a properly designed H stab preventing an elevator deflection from stalling (your favorite very large aircraft).
That is the relationship between rudder and V stab.  Without the V stab, a deflected rudder will yaw the plane until whatever is on the right side (usually the fuselage) creates enough counter force to stop the yaw.
In the extreme case, with a very large rudder, the plane will yaw until the rudder lines up with the wind.  Note if the plane is spinning, the deflected rudder never can line up with the wind.  So this is why a fully deflected rudder into a spin is not a good idea.
But the V stab and rudder, properly designed, do not "fight" each other, they simply do their jobs.
For more manuverability, have a look at the ones on a Bf 109.  Around 2/3s rudder, low aspect, swept leading edge, will continue working at higher AOA.  Now the Cessna 172:  a bit more conservative, good swept leading edge, not quite as much rudder.  Better for a trainer.


text:
In the US, would it ever be legal to fly a loop in any airplane or glider that was operating in the "utility" rather than "acrobatic" category?
1) In the US, would it ever be legal to fly a loop in any US-registered airplane or glider that was operating in the "utility" category rather than the "acrobatic" category?  (Assuming that no special waiver has been issued by the FAA: the question is not about Bob Hoover's famous Aero Commander routine etc.)  Among other things, please consider the possibility that a loop may be listed as an approved maneuver in the "utility" category in the aircraft manual, which may or may not have originated in the US.
2) And here's a closely related, but more specific question, that I'd also like answered: Is it accurate to say that inside loops are generally permitted while flying in the "utility" category in the DG-1000S sailplane in Germany, but not while flying a US-registered DG-1000S in the US in the utility category?"
This FAR seems highly relevant:

§91.9   Civil aircraft flight manual, marking, and placard
  requirements.
(a) Except as provided in paragraph (d) of this section, no person may
  operate a civil aircraft without complying with the operating
  limitations specified in the approved Airplane or Rotorcraft Flight
  Manual, markings, and placards, or as otherwise prescribed by the
  certificating authority of the country of registry.

Some context-- note the extensive discussion of "semi-aerobatic" gliders here.    I don't know whether said gliders are certified in the "acrobatic" category or not; certainly there are some aerobatic maneuvers that they are not cleared to do.
https://groups.google.com/forum/m/#!topic/rec.aviation.soaring/3UgaY9ydB6o
This link to a US glider club website suggests the answer to the first question is "yes" --  unless someone is just suggesting it's ok to be lax with the regulations-- the statement is made that it is "approved" to loop some gliders -- specifically including the DG-1000S-- when flown at a weight that puts them in the "utility" category, while at a lighter weight the same gliders would be in the "aerobatic" category and would be approved for more maneuvers.
quoting:

Approved Maneuvers in Utility Category
DG-1000:
  ● G.W. below 1650 lbs
  ● No water ballast
  ● Tail ballast used
  to compensate for rear pilot OK
Approved Maneuvers:
  ● Spins
  ● Inside Loop
  ● Chandelle
  ● Lazy Eight (Wingover)
  ● Stall Turn (Hammerhead)
More advanced requires Aerobatic category

(From https://www.flybasa.org/s/basic-aerobatics.pdf )
More information: the English version of the factory-produced flight manual for the German-made DG-1000S glider (may be downloaded here) states the following--

2.6 Approved manoeuvres Category „Utility“: The glider is certified for normal gliding in the "Utility" category. Simple aerobatics are
  approved but only without waterballast and with the weight of the rear
  pilot compensated by ballast in the ballast box in the fin see section
  6.8.7.
The following aerobatic manoeuvres are approved with all spans:
Spins
Chandelle
Inside loop
Turn
Lazy Eight
Recommended entry speeds see section 4.5.8.1.

The manual then goes on to describe additional maneuvers allowed when flown at a lower weight limit, in the "aerobatic" category:

In addition to the manoeuvres in category „Utility“ the following manoeuvres are approved:
Inverted flight
half flick roll from normal to inverted flight with half loop
half loop and half roll
half flick roll from inverted to normal flight
half roll and half loop
slow roll

This link to an Australian aerobatics association website suggests that in that country, for any given aircraft, the pilot's handbook or cockpit placards will list all the maneuvers allowed in the Utility category, while in the Aerobatic category all standard aerobatic maneuvers are assumed to be permitted unless specifically listed as not allowed in the pilot's handbook or cockpit placards:
http://www.aerobaticsaustralia.com.au/ACC_WP/pilot-briefing/casa-guidelines/
But this link to a related A.S.E. answer suggests the answer to the first question in the US is "no"--
What are the US definition and restrictions on aerobatic flight?
So, please help me sort this out--
If your answer contains any answers to these additional closely related questions, it would be appreciated:
3) What, if any, FARs other than 91.9 are involved in the answer to question (1)?
4) In the US, is the situation in regard to question (1) fundamentally different for gliders than for airplanes for any reason?
5) What if a US-registered aircraft, and its flight manual, predates the establishment of the "utility" and "aerobatic" categories?  Is it then up to the pilot's discretion whether to loop it or not in the US?

I should know this but I don't--

Not, not safe. Probably voids your insurance if you kill yourself doing it too and there are witnesses. Find a plane that known to be good (certified) for those maneuvers.

Is it possible that a utility category aircraft MUST be approved for certain listed maneuvers, but MAY be approved for others as well such as a loop?

Additional links of very peripheral interest, not related enough to really belong in the question--
https://www.aopa.org/news-and-media/all-news/2002/july/flight-training-magazine/legal-briefing
https://www.aopa.org/news-and-media/all-news/2000/may/pilot/loops-spins-and-rolls
https://www.pilotsofamerica.com/community/threads/normal-aerobatics.41793/

Related

Please consider settling on a final version for this question.

@Federico -- OK I will leave it as is as long as it is not considered too broad w/ all the various sub-questions.

This question is related but may only apply to aircraft certificated in the US; also it is not clear whether an aircraft certificated in the US may have other maneuvers added to the basic default ones for the "utility" category. https://aviation.stackexchange.com/questions/12227/what-are-the-regulatory-categories-of-aircraft-defined-by-the-faa

This Schweizer 1-26 manual says the glider is "fully aerobatic" with no reference to "utility" or "acrobatic" categories, I suppose because it predated those inventions.  I know Schweizer has issued sort of an advisory retraction of the aerobatic status which may or not be legally binding (regulatory), but if they hadn't I suppose loops would be allowed in the 1-26 as far as the FAA is concerned? http://www.harrishillsoaring.org/doc/sgs1-26manual.pdf

Also, the question is not intending to ask about EMERGENCY situations; hope that was more or less implied; I'll avoid making yet another edit.

related, narrower question asking only about US-certified aircraft -- https://aviation.stackexchange.com/questions/66644/are-loops-or-rolls-allowed-in-the-utility-category-in-any-us-certified-airplan

Here's what the Cessna 172S POH says.  If you attempt any aerobatics outside of this, you are pretty much a test pilot. Nothing here about loops.
UTILITY CATEGORY
SECTION 2
LIMITATIONS
This airplane is not designed for purely aerobatic flight. However,
in the acquisition of various certificates such as commercial pilot
and flight instructor, certain maneuvers are required by the FAA. All
of these maneuvers are permitted in this airplane when operated in
the utility category.
In the utility category, the rear seat must not be occupied and the
baggage compartment must be empty.
UTILITY CATEGORY MANEUVERS AND RECOMMENDED ENTRY SPEED*
Chandelles ................................. 105 Knots
Lazy Eights ................................. 105 Knots
Steep Turns ................................. 95 Knots
Spins ............................... Slow Deceleration
Stalls (Except Whip Stalls) ................ Slow Deceleration

Abrupt use of the controls is prohibited above 98 knots.
Aerobatics that may impose high loads should not be attempted.
The important thing to bear in mind in flight maneuvers is that the
airplane is clean in aerodynamic design and will build up speed
quickly with the nose down. Proper speed control is an essential requirement
for execution of any maneuver, and care should always
be exercised to avoid excessive speed which in turn can impose excessive
loads. In the execution of all maneuvers, avoid abrupt use
of controls.

The answer is a useful data point so thanks.  The intent of the question is to ask whether there are any cases where it is ok (legal), not to ask whether it is legal in all cases; hope you don't feel latest edit (use of word "any")  invalidates your answer, if so guess I could roll back.

In an emergency, the pilot in command can do whatever is needed to resolve the emergency.  Diving down to put out an engine would build up speed, then when the fire is out, you need to pull out of the dive & land. Would you need to perform a loop for that? Doubtful. I can't think of any reason why a loop would need to be performed.

The intent is to ask if there are cases where it is fine ( compliant with US regs) to take off with the intention of performing loops while operating in utility category- perhaps because loops are listed as allowed in utility category in aircraft manual or on placards.  Not really intending to ask about emergencies.

In "The Handbook of Glider Aerobatics", dwnldble here https://s3-eu-west-1.amazonaws.com/bga-sg-archive/Books/THE+HAND+BOOKOF+GLIDER+AEROBATICS.pdf, table 2 on page 31 lists 9 differnt glidrs incldng K13/K10/K8, K6e, K6cr, Pirat, St'd Jantar, ASW19 and GrobG102 tht are suitable for loops but NOT for rolling maneuvers.  Whether these gliders -- the newer ones that don't predate the whole "category" system-- are in the "utility" or "aerobatic" category when flown at their most favorable CG position and loading, I have no idea.

Likewise I have no idea whether it would be legal to loop all these 9 gliders types in the US in US-registered aircraft.

I was at the National Soaring Museum in Elmira, NY, over the weekend. Some neat looking planes there, including older gull wing planes.  A competition & show was going over the weekend, would have gotten there sooner had I known. Didn't occur to me to ask about loops & rolls, we got there kind of late, a little before closing.

As noted in the question, the flight manual for the German-made DG-1000S sailplane specifically states that loops are permitted in the "utility" category.  More advanced maneuvers are permitted at a lighter loading, when the glider is in the "utility" category.
In "The Handbook of Glider Aerobatics", downloadable here , table 2 on page 31 lists 9 different gliders ( K13/K10/K8, K6e, K6cr, Pirat, Standard Jantar, ASW19 and Grob G102 (non-acro)) (none of which are or were manufactured in the US) that are cleared for loops but not rolling maneuvers.  While some of these aircraft may predate the whole "category" system, it seems inconceivable that any are actually in the "aerobatic" category at any loading or CG position.  For example, I have looked at the flight manual for the Grob 102 Astir CS downloadable here, which does state that loops are allowed, and there is no long list of prohibited maneuvers, as would surely need to be the case if the aircraft were actually in the "aerobatic" category but not allowed to fly any rolling maneuvers.  (The flight manual for the Grob 102 "Astir" actually makes no reference whatsoever to "category", be it "normal", "utility", "aerobatic", or other.)
I strongly suspect that any maneuver specifically listed by an aircraft's flight manual as permitted in the "utility" category, is in fact legal to fly in the US, at the "utility" loading, in a US-registered example of that aircraft.  Clearly in at least one instance (the DG-1000S sailplane) this would include inside loops.
I also strongly suspect that any maneuver listed as permitted in the flight manual of an aircraft that predates the whole "category" system, is in fact legal to fly in the US in a US-registered example of that aircraft, even if not registered in the "aerobatic" category.
I'm aware of no FAR that explicitly states that a US-registered aircraft must be being listed and flown in the "aerobatic" category in order to be looped, even if the flight manual permits loops to be flown in other categories, or even if the flight manual predates the "category" system.
So the answers to the questions appear to be:

1) In the US, would it ever be legal to fly a loop in any
  US-registered airplane or glider that was operating in the "utility"
  category rather than the "acrobatic" category?

Yes, if the loop is listed as an approved maneuver in the utility category, as is sometimes the case.

2) And here's a closely related, but more specific question, that I'd
  also like answered: Is it accurate to say that inside loops are
  generally permitted while flying in the "utility" category in the
  DG-1000S sailplane in Germany, but not while flying a US-registered
  DG-1000S in the US in the utility category?"

No

3) What, if any, FARs other than 91.9 are involved in the answer to
  question (1)?

Unknown at this time

4) In the US, is the situation in regard to question (1) fundamentally
  different for gliders than for airplanes for any reason?

Not in a regulatory sense.  In a practical sense, the difference is that gliders due to their long wingspans typically have a rather low roll rate.  To design a glider that can roll well requires a big sacrifice of soaring performance.  This is not true of looping, especially in the case of a glider that is designed to tolerate the load of heavy water ballast, but will only be looped without water ballast.  So it's not surprising that we see many gliders that are cleared for loops but not rolls. It's more logical to accommodate this by adding loops to the list of maneuvers that are allowed in the utility category, than to say that the glider is in the aerobatic category but with a very long list of prohibited maneuvers.  (Note also that a rolling capability really is required to safely exit other maneuvers such as sustained inverted flight--especially after a piloting error resulting in some excess buildup of airspeed-- which might otherwise be perfectly within the capability of many gliders.) Conversely, with a typical light plane, it is easier to roll the plane then to loop the plane, so I'm not sure why we never see aileron rolls added to the list of allowed maneuvers in the utility category for some airplanes.

5) What if a US-registered aircraft, and its flight manual, predates
  the establishment of the "utility" and "aerobatic" categories? Is it
  then up to the pilot's discretion whether to loop it or not in the US?

The complete answer is unclear but any maneuvers specifically listed as permitted in the flight manual would likely to be allowed to be performed in the US in a US-registered example of that aircraft.

This answer could be improved by finding more authoritative sources.  Or another better answer could be posted that relied on more authoritative sources.

In particular the issue of the fact that the flight manuals produced above were not for us-manufactured aircraft, so would maneuvers listed in the "utility" category still be legal to be flown in the US for US-registered examples of those aircraft, has not been completely settled.  But I see no FAR that says "an aircraft must be in the aerobatic category in order to perform a loop."